The present invention relates to the foaming or expansion of thermoplastic resin compositions and, more specifically, to foamed processes, foamable compositions and foamed articles involving a thermoplastic resin composition containing from about 0.01 to about 5 percent by weight, based on said thermoplastic resin, of at least one compound effective to suppress the escape of the blowing agent gas from the expanded foam, with a resultant improvement in the gas efficiency of the blowing agent together with a substantial reduction in the shrinkage of the expanded foam and an improvement in its elevated temperature dimensional stability.
Generally, thermoplastic resin foam is produced by adding a blowing agent to the resin and then heating the resultant resin mixture to gasify the blowing agent for expanding the resin. Thus, improvement in the efficiency of such blowing agents is very significant for commercial-scale production of foams.
Among thermoplastic resins, polyolefin resins are particularly difficult to expand directly in a commercial-scale process. This has been attributed to the high degree of temperature dependence of their thermo-rheological viscoelasticity and to the high gas permeability of their foam cell walls. Further complications arise from endothermic expansion and crystallization which may occur during the foaming process. Commonly, especially for expansion ratios of 5 or more, polyolefin resins are modified prior to expansion, by crosslinking or blending with other resin or resins.
Japanese Patent Publication No. 35(1960)-4,341 discloses extrusion-foaming of a noncrosslinked polyolefin resin using such a relatively expensive blowing agent as 1,2-dichlorotetrafluoroethane. However, the resulting foam product often undergoes a gradual shrinkage and a density change and has a poor surface appearance with many wrinkles. Also, it undergoes a marked shrinkage when the ambient temperature is higher than about 70.degree. C. Thus, improved quality and dimensional control of the polyolefin resin foams produced by such a process are desirable.
More recently, a variety of chemical additives have been discovered which improve the quality and dimensional stability of thermoplastic resin foams, especially olefin polymer foams, without the necessity of crosslinking and/or of using resin blending techniques while permitting the satisfactory utilization of less expensive volatile organic blowing agents in place of 1,2-dichlorotetrafluoroethane. More specifically, such additives include saturated higher fatty acid amides, saturated higher fatty acid amines and complete esters of saturated fatty acids as disclosed in Watanabe et al., U.S. Pat. No. 4,214,054, partial esters of long chain fatty acids as disclosed in Cronin, U.S. Pat. No. 3,644,230 and Ehrenfreund, U.S. Pat. No. 3,755,208; partial and complete esters of aliphatic fatty acids as disclosed in Komori, U.S. Pat. No. 4,217,319; and the related technology of Japanese Kokai Nos. 53-102,971, 54-34,374 and 54-39,476.
The present invention provides another group of such chemical additives which are effective to improve the dimensional stability of thermoplastic resin foams, especially, aliphatic olefin polymer foams. More specifically, the present invention provides a foamable thermoplastic resin composition which is easily expanded using commonly available inexpensive blowing agents to give a foam product exhibiting reduced shrinkage and improved dimensional stability, especially, elevated-temperature dimensional stability, as well as improved surface smoothness.